The present invention relates to liquid crystal display devices, power supply circuits for use in liquid crystal display devices or other devices, and methods for controlling liquid crystal display devices. In particular, the present invention relates to active-matrix-type liquid crystal display devices.
In recent years, liquid crystal display devices have been widely used as screens of TVs and personal computers, panels of mobile equipment, and others.
FIG. 8 is a diagram schematically showing a configuration of a conventional liquid crystal display device. Hereinafter, an active-matrix-type liquid crystal display device will be described.
As shown in FIG. 8, the conventional liquid crystal display device includes: a liquid crystal panel 101 including pixels (not shown) arranged in a matrix pattern; a source driver 102 for controlling the gray-scale levels of the pixels in the liquid crystal panel 101; and a DC-DC power supply 103 for driving the source driver 102 by supplying current to the source driver 102. Although not shown, a gate driver for switching the pixels in the liquid crystal panel 101, a controller for supplying a control signal to the source driver 102 and other components are also provided.
In the liquid crystal panel 101, liquid crystal is sandwiched between two opposed electrodes. One of the electrodes is connected to a thin film transistor (TFT) for each of the pixels. The gate of the TFT receives a voltage from the gate driver. Control of the voltage applied to the gate allows switching operation to be performed on each of the pixels. The source of the TFT receives a voltage from the source driver 102. Control of the voltage applied to the source allows the gray-scale levels of the pixels to differ from one pixel to another. The source driver 102 receives a signal from the controller and a current from the DC-DC power supply 103. A voltage from the DC-DC power supply 103 may be applied to the drain of the TFT (i.e., common electrode).
In the DC-DC power supply 103, an operational amplifier 104 for amplifying an input voltage is provided. The operational amplifier 104 may be connected to a booster (not shown). The booster is used to boost a reference voltage supplied from an external power supply and to supply the boosted voltage to the operational amplifier 104.
However, the conventional liquid crystal display device has the following drawback.
The DC-DC power supply 103 supplies a constant current Imax to the source driver 102. The amount of this current Imax is sufficient for allowing the source driver 102 to operate at the maximum output. It was believed that the supply of such a sufficient amount of current allows the liquid crystal panel 101 to display a stable image. However, in fact, a large amount of current is constantly generated irrespective of display patterns on the liquid crystal panel 101, resulting in high power consumption in the DC-DC power supply 103 and the source driver 102.
Such a drawback is common to power supplies for supplying voltages to electrodes in the liquid crystal panel 101.